BCL-2 family proteins are key regulators of apoptosis, a form of programmed cell death essential for normal development and tissue homeostasis. Deregulation of the apoptotic machinery can lead to a host of human diseases characterized by too many or too few cells. Pathologic enforcement of cell survival by anti-apoptotic BCL-2 proteins can lead to the development, maintenance, and chemoresistance of human cancer. Anti- apoptotic MCL-1 has emerged as a major oncogenic factor in a broad range of human cancers. Essential to normal development and hematopoiesis, MCL-1 can bind and block a host of pro-apoptotic BCL-2 proteins and is subject to exquisite regulation both at the protein and RNA levels. MCL-1 also has a variety of alternative splice forms whose roles are largely unknown. Paradoxically, a shortened splice form, termed MCL-1S, retains only the BCL-2 homology 3 (BH3) domains, the conserved death domain that confers killing activity to pro- apoptotic members. Indeed, MCL-1S has been reported to trigger apoptosis when overexpressed and, ironically, the Walensky laboratory has found that the BH3 domain of MCL-1 is a potent and exclusive inhibitor of MCL-1, suggesting a regulatory role for MCL-1S in modulating the anti-apoptotic activity of full-length MCL-1 (MCL-1L). Here, I propose to dissect the mechanism underlying the paradoxical pro-apoptotic activity of MCL- 1S, with the goal of harnessing the MCL-1S pathway to overcome MCL-1 mediated chemoresistance in human cancer. Specifically, I aim to: (1) Define the spectrum of pro-apoptotic stimuli that operate through MCL-1S; (2) Dissect the pro-apoptotic mechanism of action of MCL-1S using novel protein capture reagents that mimic it's alpha-helical BH3 effector domain; and (3) Harness the MCL-1S pathway to reactivate apoptosis in the context of MCL-1L-mediated chemoresistance. I am eager to undertake multidisciplinary training at the interface of chemistry, apoptosis biology, and cancer medicine. My proposed graduate training plan at the Dana-Farber Cancer Institute and Harvard Medical School offers state-of-the-art resources, world class faculty advisors and colleagues, and an outstanding environment to continue my scientific development. I am committed to a scientific career focused on characterizing the protein interaction dynamics of the cell death machinery and harnessing these insights to advance novel strategies for subverting the apoptotic blockades of refractory human cancer. PUBLIC HEALTH RELEVANCE: BCL-2 family proteins regulate programmed cell death, or apoptosis, to maintain the essential balance between new and dying cells during development and homeostasis. Cancer cells hijack this natural pathway by overexpressing anti-apoptotic members, such as the full-length form of MCL-1 (MCL-1L), which has emerged as one of the most common oncogenic factors in human cancer. In this proposal, I will use unique chemical tools, cell survival analyses, and mouse models of human lymphoma to investigate how a short pro-apoptotic splice form of MCL-1, termed MCL-1S, neutralizes MCL-1L activity, with the goal of advancing a novel therapeutic strategy based on MCL-1S activity to overcome MCL-1L-mediated chemoresistance.